FIELD OF THE INVENTION
The present invention relates to an ultrasonic diagnosing apparatus for measuring a moving velocity of a moving object, such as a blood in a living body, and more particularly to an ultrasonic diagnosing apparatus for displaying a blood stream in superimposition on an ultrasonic image.
An ultrasonic diagnosing apparatus for detecting a distribution of a blood stream by utilizing the Doppler effect and displaying the blood stream in superimposition on an ultrasonic image has been known as a color Doppler apparatus and has been widely used. In principle, in such a color Doppler apparatus a pulsatory ultrasonic beam is emitted from an ultrasonic vibrating element array with a constant time interval. A time period from an instant at which the ultrasonic beam is emitted toward an object to a timing at which an ultrasonic wave reflected by the object is received by the vibrating element array to produce an echo signal is measured and a variation in frequency of a received signal is measured to detect a position and a movement of the object. Initially the color Doppler apparatus was utilized to diagnose the circulatory system of the human being such as the heart, but recently it has been used to diagnose other organs such as the stomach due to the fact that the color Doppler apparatus can provide a large amount of information.
FIG. 1 is a block diagram illustrating the known color Doppler apparatus. A reference numeral 101 denotes a vibrating element array, 102 denotes vibrating elements, 103 delay circuits, 104 denotes a switching circuit, 105 denotes a signal generator, 106 denotes a pulse generator, 107 summing denotes a circuit, 108 denotes receiving amplifier, 109 band pass filter (BPF), 110 denotes 90-degree phase shifter, 111a, 111b denote multiplying circuits, 112a, 112b denote low pass filters (LPF), 113a, 113b denote A/D converters, 114 denote a MTI filter for extracting a blood stream component, and a reference numeral 115 represents an autocorrelation circuit for detecting a frequency component.
An output signal of the signal generator 105 is supplied to the pulse generator 106 to produce a pulse having a center frequency f.sub.0. The pulse is then supplied to the vibrating element array 101 by means of the switching circuit 104 and delay circuit 103. By suitably operating the switching circuit 104, respective vibrating elements 102 of the array 101 are energized such that an ultrasonic beam 116 is emitted in a given direction. The ultrasonic beam 116 is reflected by red blood cells contained in blood flowing through a blood vessel 117 and is received by the vibrating elements 102 and is converted into echo signals. The echo signals are supplied to the summing circuit 107 by means of the delay circuits 103 and switching circuit 104 and are summed up thereby. An output signal of the summing circuit 107 is amplified by the receiving amplifier 108 to a suitable level and then is supplied to BPF 109 to cut off noise other than a desired frequency component. Then, the output signal of BPF 109 is supplied to an orthogonal detector, constituted by the multiplying circuits 111a and 111b and 90-degree phase shifter 110, to detect a difference frequency component .DELTA.f between the orthogonally detected output signal and the reference signal generated by the signal generator 105.
The difference frequency .DELTA.f represents the Doppler frequency of the blood stream in the blood vessel 117 and is dependent upon an angle between the ultrasonic beam 116 and the blood vessel 117. That is to say, the Doppler frequency represents a component Vd of the blood stream velocity V in the direction of the ultrasonic beam. In this manner, the Doppler frequency depends largely upon the angle between the ultrasonic beam 116 and the blood vessel 117 under inspection.
The output signals from the multiplying circuits 111a, 111b are supplied, via LPFs 112a, 112b, to A/D converters 113a, 113b and are converted into digital signals. Then, the digital signals are supplied to the MTI filter 114 and a DC component (clutter component), which corresponding to echo signal components reflected from stationary tissues, is removed to extract a signal component representing an echo signal component reflected from the blood stream. The thus derived blood stream component is supplied to the autocorrelation circuit 115 to detect the above mentioned difference frequency component .DELTA.f. This component is then displayed on a display monitor by means of a digital scan converter. The display monitor displays a B-mode ultrasonic image representing the strength of the ultrasonic wave reflected by the stationary tissues in the living body in superimposition therewith. The blood stream signal is displayed with a special color such as red and blue in contrast with the B-mode ultrasonic image. In the known color Doppler apparatus, the Doppler component fluctuates greatly, and thus the ultrasonic beam is emitted in the same direction about ten times and an average of the detected blood stream components is derived in order to attain a necessary measuring accuracy.
In the known color Doppler apparatus explained above, it is difficult to separate the blood stream signal from the clutter component reflected from the stationary objects when the blood stream flows at a relatively low velocity, so that the measurement could not be carried out practically. The separation between the Doppler signal and the clutter component is performed by means of the MTI (moving-target indicator) filter 114 which is constructed by digital circuits. FIG. 2 shows a response characteristic of the MTI filter 114. As illustrated in FIG. 2, the response of the MTI filter could not be steep enough to remove the clutter component sufficiently and when MTI filter is constructed to remove the clutter component sufficiently, the blood stream signal is also suppressed to an undesired extent. In this manner, the known color Doppler apparatus could not measure the blood stream of a low velocity. Further the Doppler effect appears only in the direction in which the ultrasonic beam 116 is emitted, and therefore it is theoretically impossible to detect the blood stream flowing in a direction perpendicular to the ultrasonic beam direction.
The ultrasonic beam is composed of a plurality of ultrasonic waves, which are emitted from respective vibrating elements 102 at different timings determined by delay times of the delay circuits 103. In a long range in which a distance between the vibrating element array 101 and the object is long, angles between lines normal to respective vibrating elements and lines connecting respective vibrating elements and the object are substantially same. However, in a short range in which the distance is small, these angles vary largely. The Doppler effect depends largely upon these angles, and thus when the distance is small, a large fluctuation is produced in the Doppler frequency and the measuring accuracy becomes very low. This problem could not be resolved by increasing the spatial resolution in the short range by utilizing dynamic focus.
Further, in the known color Doppler apparatus mentioned above, in addition to a usual ultrasonic pulse transmitting sequence, there is set a special sequence in which an ultrasonic pulse having a wide pulse width is transmitted for detecting a blood stream having a small velocity. Moreover, in order to increase the accuracy of the Doppler measurement, it is necessary to repeat the Doppler sequence by about ten times and an averaging process is required. Therefore, the frame rate of the color Doppler apparatus is decreased greatly as compared with the usual B-mode ultrasonic image and the spatial resolution is liable to be decreased.